In the case of blade coaters, substantially two running modes of different types are known. On one hand, the so-called large-angle coating is known, in which the coating blade normally forms an angle of several dozens of degrees with the moving base to be coated. On the other hand, the so-called small-angle coating is known, in which the coating blade forms an angle of just a few degrees (normally from about 0.degree. to about 10.degree. ) with the moving base to be coated.
The present invention is related to the latter, small-angle blade coating and in particular to a zero-angle application of the small-angle blade coating, i.e., to a case in which the blade angle at the tip of the coating blade is less than 0.degree., i.e., the tip of the coating blade has been bent over from 0.degree., so that the smallest gap between the blade and the coating base is placed before the tip of the blade.
In view of the profile of the coating quantity and in view of keeping the blade tip clean, the best results have been obtained exactly with the zero-angle blade coating, especially when the coating base is a roll face in a size press. A drawback of this prior art method has, however, been a highly limited range of operation with respect of the range of coating quantity to be controlled for if the blade has been loaded excessively in an attempt to reduce the coating quantity, the blade tip has been bent excessively apart from the coating base, which has resulted in streaks in the coating. Such a coating device that makes use of the prior art coating method is illustrated schematically in FIG. 6. In this figure, the coating device is denoted generally with the reference numeral 30. The coating device 30 as shown in FIG. 6 is a so-called short-dwell coating device, which comprises a pressurized coating-agent chamber 31, which is defined by the coating blade 32, by a front seal 33, and by lateral seals (not shown). The coating blade 32 is installed in a blade holder 34, and the coating blade 32 is loaded against the moving base B, such as the face of a roll in the size press, by means of a loading hose 35 in the area between the blade holder 34 and the tip 39 of the coating blade. The loading hose 35 is installed in a loading-hose holder 37, with which an adjusting device 36 is also connected, by whose means the loading-hose holder 37 can be adjusted in the coater. In FIG. 6, the coating distance, i.e., the coating zone, is denoted with the reference S1 and the zone is defined in the area between the coating blade 32 and the front seal 33, in which the coating agent is in direct contact with the moving base B to be coated. The direction of movement of the moving base B is denoted with an arrow and with the reference D.
In the prior-art solution shown in FIG. 6, the loading of the coating blade 32 has been increased to such a high level that the tip 39 of the coating blade has been bent apart from the base B to be coated, which has produced cavitation at the tip 39 of the coating blade, as a result of which streaks have been formed in the layer of coating agent formed on the face of the moving base B.